1. Field of the Invention
The present invention relates to an image forming method using a photothermographic material, and particularly to an image forming method using a photothermographic material which can be thermally developed stably and quickly.
2. Description of the Related Art
In recent years, it has been strongly desired in the field of films for medical imaging and for graphic arts to reduce the amount of used processing liquid waste in consideration of environmental protection and space saving. For this reason, technology regarding photothermographic materials as films for medical imaging and for photographic applications, which are capable of efficient exposure with a laser image setter or a laser imager and capable of forming a clear black-toned image with high resolution and high sharpness is desired. Such photothermographic materials can eliminate use of liquid processing chemicals and can provide users with a thermal development system which is simpler and does not contaminate the environment.
Although similar requirements also exist in the field of general image forming materials, an image for medical imaging requires a particularly high image quality excellent in sharpness and granularity because a delicate image representation is desirable. Also an image of blue-black tone is preferred in consideration of easy diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as ink jet printers and electrophotographic systems, are available as general image forming systems, but they are not satisfactory as output systems for medical images.
On the other hand, thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems”, written by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. A photothermographic material generally comprises a photosensitive layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt) and, if necessary, a toner for controlling the color tone of a developed silver image are dispersed in a matrix of a binder. The photothermographic material, when heated at high temperature (for example, 80° C. or higher) after image exposure, forms a black-toned silver image by an oxidation/reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizer) and the reducing agent. The oxidation/reduction reaction is promoted by a catalytic effect of a latent image formed by exposure on silver halide. Accordingly, a black-toned silver image is formed in an exposed area. Such materials are disclosed in many documents including, for example, U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924. Further, Fuji Medical Dry Laser Imager FMP-DPL is an example of a practical medical image forming system using a photothermographic material that has been marketed.
In production of a thermographic system using an organic silver salt, two methods are available. In one method, a solvent coating is adopted and in the other method a coating liquid containing polymer fine particles as a main binder in an aqueous dispersion is applied and dried. In the latter method, since no necessity arises for a process of solvent recovery or the like, a production facility is simple and the method is advantageous for mass production.
As described above, various improvements have been applied to photothermographic materials from the viewpoints of environmental considerations, cost, and photographic properties. However, there remains room for improvement in thermal developing speed. In general, quick processing of photographed images is desired in the medical field or other fields in order to obtain the results of diagnosis quickly.
In order to conduct thermal developing treatment quickly, it is necessary to increase the speed of thermal development, and Japanese Patent Application Laidpen (JP-A) Nos. 2002-156727 and 2001-264929 state that it is effective to use a development accelerator. The use of the development accelerator can increase development speed. However, in continuous thermal developing treatment of plural sheets of the photothermographic material, an increase in fogging, variation in sensitivity or variation in Dmax may be caused between images obtained at the initial stage and images obtained after the continuous treatment. Thus, improvement has been desired.
When it suddenly becomes necessary to output an image at the scene of emergency care, or some other scene, it is desired to shorten the start-up time of an image forming apparatus, which is an amount of time from a state in which a power supply of the apparatus is shut off to a state in which the apparatus can start thermal development. Conventionally, this has been attempted by quickening a rise in temperature of heater plates of a thermal developing portion by setting a temperature thereof to a temperature higher than the temperature for development immediately after the apparatus is started up, and quickly bringing the heater plate temperature close to the set temperature by monitoring and feeding back the heater plate temperature. However, there are problems in that, for example, variation in photographic properties becomes large due to large variation in temperature when hunting for the appropriate temperature. Thus, improvement has been desired.